1. Field of the Invention
The present invention relates to portable induction heaters.
2. Description of the Related Art
Plastic and rubber membranes are utilised for providing waterproofing to roofing structures, tunnels and tanks. In the roofing industry, the membranes are laid down upon an insulation layer of either rockwool board, plastic resin foam board or even gypsum board, which is itself laid and fixed to the main substructure, which could be concrete, wood or steel sheet. The membranes are fixed either by glue, directly to the insulation boards, as in FIG. 10, or glued to numerous small anchor pads that have been screwed to the substructure through the insulation boards, as in FIG. 11. The glue is usually a solvent based adhesive, which is toxic and unpleasant to apply and can have unacceptably long curing times in cold climate conditions. Alternatively, the membranes are fixed with screws and suitable washers directly through the edge of each sheet of membrane and into the substructure through the insulation boards. This has the disadvantage that additional steps must be taken to prevent leakage through the membrane at the points where the fastenings extends therethrough in that the next sheet of membrane laid must cover the screws and fixings and then be heat welded to the previous sheet, as in FIG. 12.
In order to overcome the aforementioned problems a rigid plastic anchor disc, as in FIG. 13, is fixed to the substructure through a pad of membrane and on through the insulation boards. The membrane pad is substantially larger than the anchor disc and has received a factory coating of heat activated adhesive mixed with a powdered metallic substance on the upper surface of the pad that is clear of the anchor disc. It is also possible that the metallic powder may be incorporated within the substance of the membrane pad at the time of manufacture. The main covering membrane overlays the anchor and membrane pad.
The powdered metallic substance is heated by an induction heater which glues the membrane pad to the overlaying membrane, thereby entrapping the plastic anchor disc and anchoring the membrane. Alternatively a metallic anchor disc can be used as in FIG. 14.
UK Patent Application No. 9506694.09 (Robertson) describes the use of an induction heater to perform the bonding operation, however it is impossible to locate the induction coil of the induction heater reliably and accurately over the metallic anchor disc or powder that requires heating (within 1 mm), because it cannot be seen underneath the membrane. Failure to guarantee accurate location causes one part of the anchor disc overheat and burn the adhesive whilst the other half is under heated and not properly bonded. Inaccurate placement could cause over-heating damage to both the adhesive and the membrane, whilst under-heating would not provide sufficient bond strength. Either of these conditions could cause the failure of the roof over an extended time scale but particularly during high wind conditions.